1. Field of the Invention
This invention in general relates to the field of optical viewfinders and in particular to camera viewfinders incorporating a viewing frame.
2. Description of the Prior Art
The application of a reverse Galilean optical system as a relatively inexpensive camera viewfinder comprising two optical elements, an objective lens and an eye lens, with magnification less than unity, is known. A field-defining aperture, or field stop, is typically provided on the flat surface of the objective lens. This configuration provides the advantages of simplicity, minimal distortion, compactness, and an erect unreversed image.
However, the reverse Galilean optical system suffers from the limitation that, because no real, intermediate image is produced in the optical path, it is not possible to provide an in-focus image of the field stop. The viewer is required to make use of a blurry frame to determine the extent of the objective field. Consequently, the precise extent of the field of view is uncertain because of the resultant inaccuracy in the process of framing, and the precise direction of the field of view changes as the frame stop appears to shift with variance in the viewer's eye position.
The relevant art discloses various optical systems for providing visual information in a viewfinder. U.S. Pat. No. 4,576,458 issued to Cho et al. discloses a camera finder comprising holograms interposed between an objective lens and an eye lens. One or more reference beams are used to illuminate the holograms and provide a field frame, with parallax correction, in the viewfinder. A configuration in which an illuminated diffractive surface is used to display information, rather than project a frame, in the field of view is disclosed in U.S. Pat. No. 4,367,463 issued to Suzuki et al. In U.S. Pat. No. 5,299,037 issued to Sakata, an illuminated diffractive surface is disposed between layers of liquid crystal materials to provide information within a viewfinder. Such viewfinder configurations, however, require a source of illumination to provide the desired display information. The components necessary to provide such illumination add to the expense of the optical system.
Thus, there remains a need for a relatively inexpensive viewfinder optical system with an in-focus frame stop, and in which the position of the frame stop on the viewed scene is not affected by changes in the viewer's eye position. It is an object of the invention to provide for such a viewfinder.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.